1. Field of the Invention
The present invention relates to channel allocation methods, wireless communication systems, and wireless communication apparatuses in wireless networks.
2. Description of the Related Art
A circuit-switching wireless transmission method, such as that used by cellular phones, sets a transmission frame period, defines a partial area of the frame period as a slot, and exclusively uses the slot for transmitting information from a communication apparatus.
In such a known wireless transmission system, a method is used in which a slot allowed to be exclusively used by a user is continuously used by that user unless the user disconnects from the line. In other words, a protocol is designed to prevent undesired interruption of using a transmission line against a user's will once a slot is allocated to be exclusively used.
The IEEE (Institute of Electrical and Electronics Engineers) has proposed the IEEE 802.11 standard, in which an optimal protocol for performing asynchronous wireless transmission is defined as a wireless LAN transmission method.
In contrast, various proposals have been made for wireless transmission systems having a periodical frame structure. For example, HiperLAN2 and Wireless 1394 have been considered. In these wireless transmission systems with a frame structure, a channel allocation transmission method for allocating in advance a channel for transmission prior to transmitting information has been used. In the channel allocation transmission method, generally channel allocation is performed in each frame period, which is defined periodically.
Referring to FIG. 18, as an example of a method for performing channel allocation in each frame period, a known case in which channel allocation is performed in only one frame will now be described. Portions (A) to (D) of FIG. 18 correspond to a first frame to a fourth frame, respectively, of a series of frames.
In FIG. 18, a predetermined transmission frame period is determined by transmitting network broadcast information B31 and B32.
A communication apparatus that wants to make a channel allocation request transmits a channel allocation request Q31 in an allocation area of the second frame (portion (B) of FIG. 18).
A network control station receives the channel allocation request Q31. If the channel can be allocated, the control station specifies a transmission area to be allocated by transmitting network broadcast information B33 in the third frame (portion (C) of FIG. 18).
The information sender, that is, the communication apparatus that has sent the channel allocation request, receives the network broadcast information B33 and transmits information using the allocated channel S31 in the third frame (portion (C) of FIG. 18) in accordance with parameters of the network broadcast information B33.
In the fourth frame (portion (D) of FIG. 18), the control station transmits network broadcast information B34, which is not specifying a transmission area to be allocated, thereby terminating the channel allocation transmission.
In known wireless transmission systems, generally information to be transmitted is discontinuous. Accordingly, the entire processing is designed to be completed by one channel allocation request.
FIG. 17 shows examples of frames in which continuous channel allocation is performed by a known method. Portions (A) to (G) of FIG. 17 correspond to a first frame to a seventh frame, respectively, of a series of frames.
Referring to FIG. 17, a predetermined transmission frame period is determined by transmitting network broadcast information B41 and B42.
A communication apparatus that wants to make a channel allocation request transmits a channel allocation request Q41 in an allocation area of the second frame (portion (B) of FIG. 17). The network control station receives the channel allocation request Q41. If the channel can be allocated, the control station specifies a transmission area to be allocated by transmitting network broadcast information B43 in the third frame (portion (C) of FIG. 17). The information sender (terminal station) receives the network broadcast information B43 and transmits information using the allocated channel S41 in accordance with parameters (transmission start/end time) of the network broadcast information B43.
In order to continuously transmit information, the information sender transmits a channel allocation request Q42 in an allocation area of the third frame (portion (C) of FIG. 17).
The network control station receives the channel allocation request Q42. If the channel can be allocated, the control station specifies a transmission area to be allocated by transmitting network broadcast information B44 in the fourth frame (portion (D) of FIG. 17).
The information sender receives the network broadcast information B44 and transmits information using the allocated channel S42 in accordance with parameters (transmission start/end time) of the network broadcast information B44. In order to continuously transmit information in the fifth frame, prior to sending the information using the allocated channel S42, the information sender transmits a channel allocation request Q43 in an allocation area of the fourth frame (portion (D) of the fourth frame).
The network control station receives the channel allocation request Q43. If the channel can be allocated, the control station specifies a transmission area to be allocated by transmitting network broadcast information B45 in the fifth frame (portion (E) of FIG. 17). The information sender receives the network broadcast information B45 and transmits information using the allocated channel S43 in accordance with parameters (transmission start/end time) of the network broadcast information B45. At the same time, the information sender transmits a channel allocation request Q44 in an allocation area of the subsequent frame in order to continuously transmit information in the subsequent frame.
The network control station receives the channel allocation request Q44. If the channel can be allocated, the control station specifies a transmission area to be allocated by transmitting network broadcast information B46 in the sixth frame (portion (F) of FIG. 17). The information sender receives the network broadcast information B46 and transmits information using the allocated channel S44 in accordance with parameters (transmission start/end time) of the network broadcast information B46. Since the information sender does not continuously send information in the subsequent frame, the information sender transmits no information in an allocation area.
In the seventh frame (portion (G) of FIG. 17), the control station transmits network broadcast information B47, which does not specify a transmission area to be allocated, thereby terminating the channel allocation transmission.
The above-described known technology has the following problems:
In circuit-switching wireless communication systems, such as those used by cellular phones, communication protocols have been designed to prevent disconnection of a wireless transmission link. A slot is not released unless a user makes a disconnection request. If such known circuit-switching communication protocols are applied to wireless LAN systems, a slot allocated to a terminal station cannot be released when a link cannot be established between the control station and the terminal station, that is, when the terminal station moves out of a communication range of the control station.
In a wireless network configured by combining a plurality of wireless communication apparatuses, one medium is occupied when a large amount of information is streaming-transmitted over a long period of time from one communication apparatus to another. In order to solve this problem, a method of setting a transmission frame period and exclusively using a partial area of the frame period for streaming transmission is devised.
With a known method such as that shown in FIG. 17, processing involved in allocating a transmission channel must be continuously performed as long as streaming transmission continues, and hence the same channel allocation request must be repeatedly made. When transmission channel allocation processing cannot be performed in this known scheme, streaming transmission is interrupted. When such a scheme is adopted by general wireless LAN systems with a frame structure, channel allocation must be continuously performed in frame units.
In wireless LAN systems, a link between the information sender and the information receiver may become unstable due to the positional relationship between the two apparatuses, the distance between the two apparatuses, and the occurrence of an obstacle, and hence a channel may not be temporarily allocated. In such a case, often times streaming transmission may not be performed. Even when streaming transmission starts, it may be frequently interrupted.
In FIGS. 17 and 18, examples in which a frame period set by a beacon is constant have been illustrated. As disclosed by PCT International Publication No. WO98/09469, the following problem occurs when a beacon transmission interval (frame period) is variable in order to prevent wireless beacons transmitted from a base station from colliding with one another. Specifically, the frame length changes every time. For example, it is desirable to have a channel of 20 Mbps in order to perform streaming transmission. However, since the frame period is short, it is impossible to allocate a time slot operating at 20 Mbps in one frame. In such a case, data is transmitted in a plurality of frames. If a transmitting terminal and a receiving terminal are not informed of information indicating such regulations, data transmission/reception time can only be determined every time after a beacon is heard. When data is transmitted in a plurality of frames, the receiving terminal needs to perform processing such as temporarily storing the received information in a buffer.